1. Field of the Invention
The present invention relates to a digital signal recording apparatus for recording digital signals to a recording medium, and particularly for recording compressed television signals containing an MPEG signal or other video signals compressed by means of a high efficiency compression algorithm, to a digital signal reproducing apparatus for reproducing the digital signals from the recording medium, and to a digital signal recording and reproducing apparatus for recording and reproducing the digital signals.
2. Description of the Prior Art
High efficiency digital coding technologies for video and other images signals have progressed significantly in recent years. Particular attention has been given to inter-frame predictive coding techniques as a means of reducing redundancy on the time-base axis, and international standards for video signal coding techniques have been established. One of these is the ISO-13818 specification, which is commonly known as MPEG-2 for the Motion Picture Expert Group of the ISO.
This method reduces redundancy on the time base by inter-frame predictive coding, and reduces redundancy on the spatial base by discrete cosine transformation (DCT) coding. Predictive error is reduced by motion compensation during inter-frame prediction to improve coding efficiency. The coded data is generally recorded using variable length block units, macro block units, MBS units, picture units, and group-of-picture (GOP) units.
Systems for combining and packetizing the video and audio coded data in an MPEG signal have been established as a means of transmitting such variable length coded data. In this system, the coded data is first converted to variable length packets called a "packetized elementary stream" (PES); these PES packets are then divided into fixed length packets called the "transport stream packets" (TSP). Auxiliary information used, for example, to synchronize the audio and video or to identify multiplexed programs is also written to the TSP.
As described in An experimental study on a home-use digital VCR (IEEE International Conference on Consumer Electronics, Jun. 6-9, 1989; WPM8.6), when recording a base band signal in a digital video cassette recorder (DVCR), the input video signal is compressed, then an error correction signal is added to the compressed video signal for coding the signal for recording, and the coded signal is then recorded on a magnetic tape. During reproduction, the signal read from the magnetic tape is detected and processed for error correction, and then decoded to output the video signal. Note that the video signal is recorded to the magnetic tape in fixed length packet units.
When an MPEG or other compressed television signal is recorded and reproduced, the signal is packetized in units of a fixed number of bits in the transmission sequence of the signal sent from the encoder. During reproduction, the readout packets are reconnected in the sequence read to reproduce the original signal, which is then transmitted to the decoder.
With this conventional method, however, the following problems develop when the input data is data compressed by inter-frame coding as in the case of MPEG signals.
Specifically, during fast forward, fast reverse and other special reproduction modes, a part of the recorded data can be lost, resulting in significant image deterioration, because the heads scan across the recording tracks.
In addition, during special reproduction modes, the data for plural frames is captured to generate the special reproduction frames, and therefore the data for plural frames may be mixed into one special reproduction frame, again resulting in significant image deterioration.
Furthermore, it is not possible to reproduce directly from the recording tracks the auxiliary signals required for the decoder to begin the decoding operation even if it is possible to scan the frame data required for frame decoding during a special reproduction operation. On DVCR without a built-in decoder, it is also necessary to transfer to the decoder the data required for decoding, but the data required for the transfer is also lost.
It is furthermore not possible to output the audio signal or text, symbol, and other character data during a special reproduction operation.
When a tape recorded with a multiplexed signal containing plural channels is to be reproduced in a special reproduction mode, it is also only possible to generate a signal multiplexed with the plural channels mixed.
It is also necessary to add a staff signal for recording rate adjustment to the input signal in order to maintain a constant recording rate during recording of a variable length coded input signal in which the amount of data transferred per unit of time varies. During reproduction, however, it is difficult to separate and reproduce the staff signal and the original input signal.
The conventional apparatus is involved with another problem that undesirable mismatching may occur when errors occur and when overwriting data on a previous recording.
For example, when an error occurs, the error will affect the range at the very least the transport stream packet (TSP) and the packetized elementary stream (PES) packet in which the error occurred. Conventionally, however, it is difficult to determine the TSP divisions in the recording packets and the divisions between the PES packets recorded on the recording medium, and this makes it impossible to restrict the range affected by the error. To determine the locations of the transmission packet divisions, an operation is required for sequentially decoding the bit stream from the header portion. When overwriting previous data, the position from which overwriting begins is irrespective of the divisions between the previously recorded packets, and signal components cannot be decoded and therefore image deterioration-remains on the tape.
Similarly, because it is not possible to determine the hierarchical divisions between, for example, sequences, GOP, and pictures, in the video signal, errors and overwriting result in occurrence of image disruption. In coding methods such as MPEG using inter-frame predictive coding, image deterioration is caused over plural subsequent frames whenever the frame serving as the basis for the predictive coding is disturbed.
Similarly, when plural multiplexed programs are contained in the input signal, it is also difficult to determine the divisions between the programs, and the effects of errors and overwriting affect over the plural programs.
Moreover, there has been a further problem that correct synchronization is not attained when the timing must be synchronized with the input signal. For example, the transmission packets of MPEG2 signal also contain such timing reference signals as a program clock reference (PCR) signal for an external decoder to synchronize the decoding operation with the clock used during the coding operation; a decoding time stamp (DTS) for synchronizing a video signal and audio signal when in decoding operation; and a presentation time stamp (PTS) for synchronizing the decoded video signal and audio signal when the image signal is displayed.
If the tape speed is different between: the recording and the reproducing modes, the transfer timing will differ from the actual timing, and the decoder will not be able to correctly decode and display the signal. For example, if the reproduction speed is three times (3.times.) the normal speed, the transfer timing of the timing reference signal will also be 3.times. the normal rate; if the reproduction speed is 1/2, the transfer timing will also be 1/2, resulting in that the decoder buffer will either overflow or underflow.
During reverse mode reproduction at, for example, -1.times. or -3.times. the normal reproduction rate, the timing reference signal updates reversely to the normal mode reproduction rate and the data is read in reverse order. If the data is then output in the reverse mode reproduction sequence, the decoder is, of course, unable to correctly decode the data.
Furthermore, when the reproduction rate changes during reproduction mode, the reproduced data is not output while the tape speed is being changed. The timing reference data is therefore also not output, and the correct timing reference signal cannot be sent to the decoder.